A PET (Positron Emission Tomography) apparatus is an apparatus which can image the behavior of a trace substance in an object (living body) to be examined by detecting a pair of 511 keV photons (gamma rays) which fly in opposite directions upon electron-positron pair annihilation in the object irradiated with RI radiation. The PET apparatus includes a detecting section having many small photon detectors arrayed around a measurement space in which an object to be examined is placed. This apparatus detects a photon pair generated upon electron-positron pair annihilation by coincidence counting, accumulates the coincidence counting information, and reconstructs an image representing the spatial distribution of occurrence frequencies of photon pairs in the measurement space on the basis of these many pieces of accumulated coincidence counting information. This PET apparatus serves an important role in the field of nuclear medicine and the like. For example, biofunctions and the high-order brain functions can be studied by using this apparatus. Such PET apparatuses are roughly classified into two-dimensional PET apparatuses and three-dimensional PET apparatuses.
FIG. 7 is a view for explaining the arrangement of the detecting section of a two-dimensional PET apparatus. FIG. 7 shows an example of an arrangement including seven detector rings, and is a sectional view of the detecting section taken along a plane including the central axis. A detecting section 10 of the two-dimensional PET apparatus has detector rings R1 to R7 stacked between a shield collimator 11 and a shield collimator 12. Each of the detector rings R1 to R7 has a plurality of photon detectors arranged in the form of a ring on a plane perpendicular to the central axis. Each photon detector is a scintillation detector formed from a combination of a scintillator such as BGO (Bi4Ge3O12) and a photomultiplier. This detector detects photons flying from a measurement space including the central axis. The two-dimensional PET apparatus has slice collimators S1 to S6 inside the detecting section 10. These slice collimators S1 to S6 are ring-like members each of which is placed between adjacent detector rings in a direction parallel to the central axis. Each slice collimator is made of a material having a larger atomic number and larger specific gravity (e.g., lead or tungsten) and has a collimating function of shielding obliquely incident photons (gamma rays).
The detecting section 10 of the two-dimensional PET apparatus having the above arrangement can perform coincidence counting of only a photon pair flying from the nearly 90° direction with respect to the central axis owing to the collimating function of the slice collimators S1 to S6. That is, the coincidence counting information, i.e., two-dimensional projection data, accumulated by the detecting section 10 of the two-dimensional PET apparatus is limited to that obtained by a pair of photon detectors included in a single detector ring or detector rings which are adjacent to each other (or very close to each other). The two-dimensional PET apparatus can therefore efficiently remove scattered radiation produced when a photon pair generated outside the measurement space is scattered. In addition, this apparatus can easily perform absorption correction and sensitivity correction with respect to two-dimensional projection data, and hence can obtain a reconstructed image with good quantitativeness.
FIG. 8 is a view for explaining the arrangement of the detecting section of the three-dimensional PET apparatus. FIG. 8 is also a sectional view of the detecting section taken along a plane including the central axis. The arrangement of the detecting section 10 of the three-dimensional PET apparatus is the same as that of the two-dimensional PET apparatus except that the three-dimensional PET apparatus has no slice collimators. The detecting section 10 of the three-dimensional PET apparatus having this arrangement has a wide solid angle and can perform coincidence counting of a photon pair flying from a wide range as compared with the two-dimensional PET apparatus. That is, as the coincidence counting information, i.e., three-dimensional projection data, obtained and accumulated by the detecting section 10 of the three-dimensional PET apparatus, data obtained by a pair of photon detectors included in an arbitrary detector ring can be used. Three-dimensional PET apparatus can therefore perform coincidence counting of a photon pair with sensitivity five to ten times higher than that of the two-dimensional PET apparatus. As compared with the two-dimensional PET apparatus, however, the three-dimensional PET apparatus has difficulty in accurately removing the influence of scattered radiation, and hence the quantitativeness of a reconstructed image is poor.
As described above, as compared with the three-dimensional PET apparatus, the two-dimensional PET apparatus having slice collimators has low photon pair detection sensitivity but can efficiently remove scattered radiation and easily perform absorption correction and sensitivity correction. The two-dimensional PET apparatus therefore has the merit of obtaining a reconstructed image with excellent quantitativeness.